1. Field of the Invention
The present invention is directed to an improved inflatable radar reflector. More particularly, it is directed to an improved means of internally suspending a system of corner reflectors that are inherently deployed to an accurate alignment upon inflation of the inflatable radar reflector.
2. Description of the Prior Art
The facility with which lost objects and/or persons may be located and retrieved or rescued is related to the effectiveness with which a radar source signal may be reflected near the object and/or person. When the object intended to reflect the radar signal is relatively small and distant, significant difficulty may be encountered in locating either the object or, for example, lost personnel such as campers, hunters, military personnel, stranded boatmen or the like. The successful incorporation of an effective corner reflector array into an inflatable device provides a radar cross-section which is many times larger than an object the same size without the corner reflector array. This enhanced radar cross-section may be seen from long distances.
The reflector may be included in an inflatable assembly having flexible or collapsible internal corner reflectors that may be folded into an extremely small volume, such as into the pocket of a life vest, so as to be easily stored. The corner reflector array may reside in a flexible material which is resilient and not easily damaged, and which in any event may be easily repaired. The corner reflector array residing inside an inflatable envelope insures that the reflector may not be bent out of shape or otherwise functionally impaired, even during severe use. The inflatable reflector may be inflated orally or with compressed air or with lighter than air gas.
Previous U.S. patents have addressed the design of inflatable corner reflector systems. It has been suggested in U.S. Pat. No. 2,463,517 that a flexible reflector array may be constructed as a single unit having the intersecting faces of the corner reflectors sewn together or otherwise joined together. The single unit reflector would then be suspended by six rubberbands from the points of the flexible reflector array to the internal surface of an inflatable envelope. It is apparent that the forces exerted by the rubberbands upon the individual sides of the reflector array are not evenly distributed to the areas of each of the reflective surfaces. Small variations in the size, shape, and alignment of each of the reflective surfaces result in puckers, sags, twists, and angular error of the reflective surfaces upon inflation of the system. The imperfections result in a loss of radar cross-section enhancement when the error exceeds one-quarter of the wavelength of the radar signal frequency. The application intended for this referenced patent was for relatively large weather balloons for use at a time when radar systems operated in longer wavelengths, i.e. lower frequencies than many modern radar systems. The imperfections associated with this configuration were not as critical for relatively large balloons and longer wavelengths as they are for small objects and current radar systems with shorter wavelengths.
U.S. Pat. No. 3,103,662 suggests improving the construction of an inflatable radar reflector by attaching the edges of the reflector array into the seams of the outer inflatable envelope. The corner reflector array is also constructed as a single unit with the intersecting surfaces sewn together or otherwise formed. The outer edges of the array are then sandwiched into the outer surface seams to form an octahedron shaped inflatable structure. Instead of reducing the puckers and errors in the reflector as claimed, this configuration, in practice, results in very pronounced and obvious distortion of the corner reflector surfaces when inflated. The strain in the outer inflatable surface material is greater at the center of each triangular area than it is in the vicinity of the points. This results in a distortion in the shape when inflated. As the octahedron shape is inflated the center area bulges out and the points of the octahedron area drawn closer to each other. The result on the attached corner reflector arrays is to hold tension on the middle of the seam edge while placing the areas attached to the points of the octahedron in compression. This results in very pronounced twists and puckers in the reflector surfaces. The magnitude of the resulting distortion is a function of the gas pressure and is greater as the pressure is increased.
Thus, even though the basic idea of providing an inflatable radar reflector is basically sound, the prior art devices for doing so are not. The distortions caused by the construction of radar reflectors of the prior art are so great as to render them ineffective for modern-day radar systems utilizing shorter wavelengths.